The present invention relates to a process for the (co-)polymerisation of olefins using a Ziegler-Natta type catalyst, in particular to a process for the gas-phase (co-)polymerisation of olefins in a fluidised bed reactor using a Ziegler-Natta type catalyst
The present invention also relates to a process for increasing the polymerisation activity of a Ziegler-Natta type catalyst during the (co-)polymerisation of olefins using said Ziegler-Natta type catalyst, in particular during the gas-phase (co-)polymerisation of olefins in a fluidised bed reactor using said Ziegler-Natta type catalyst
The present invention further relates to a process for increasing the comonomer response during the copolymerisation of olefins using a Ziegler-Natta type catalyst in particular during the gas-phase copolymerisation of olefins in a fluidised bed reactor using a Ziegler-Natta type catalyst
Processes for the co-polymerisation of olefins in the gas phase are well known in the art. Such processes can be conducted for example by introducing the gaseous monomer and comonomer into a stirred and/or fluidised bed comprising polyolefin and a catalyst for polymerisation.
In the gas fluidised bed polymerisation of olefins, the polymerisation is conducted in a fluidised bed reactor wherein a bed of polymer particles is maintained in a fluidised state by means of an ascending gas stream comprising the gaseous reaction monomer. The start-up of such a polymerisation generally employs a bed of polymer particles similar to the polymer which it is desired to manufacture. During the course of polymerisation, fresh polymer is generated by the catalytic polymerisation of the monomer, and polymer product is withdrawn to maintain the bed at more or less constant volume. An industrially favoured process employs a fluidisation grid to distribute the fluidising gas to the bed, and to act as a support for the bed when the supply of gas is cut off. The polymer produced is generally withdrawn from the reactor via a discharge conduit arranged in the lower portion of the reactor, near the fluidisation grid. The fluidised bed consists in a bed of growing polymer particles. This bed is maintained in a fluidised condition by the continuous upward flow from the base of the reactor of a fluidising gas.
The polymerisation of olefins is an exothermic reaction and it is therefore necessary to provide means to cool the bed to remove the heat of polymerisation. In the absence of such cooling the bed would increase in temperature and, for example, the catalyst would become inactive or the bed would begin to melt. In the fluidised bed polymerisation of olefins, the preferred method for removing the heat of polymerisation is by supplying to the polymerisation reactor a gas, the fluidising gas, which is at a temperature lower than the desired polymerisation temperature, passing the gas through the fluidised bed to conduct away the heat of polymerisation, removing the gas from the reactor and cooling it by passage through an external heat exchanger, and recycling it to the bed. The temperature of the recycle gas can be adjusted in the heat exchanger to maintain the fluidised bed at the desired polymerisation temperature. In this method of polymerizing alpha olefins, the recycle gas generally comprises the monomer and comonomer olefins, optionally together with, for example, an inert diluent gas such as nitrogen or a gaseous chain transfer agent such as hydrogen. Thus, the recycle gas is used to supply the monomer to the bed, to fluidise the bed, and to maintain the bed at the desired temperature. Monomers consumed by the polymerisation reaction are normally replaced by adding make up gas or liquid to the polymerisation zone or reaction loop.
It is well known that Ziegler-Natta type catalysts can advantageously be used for the (co-)polymerisation of olefins, particularly in slurry processes as well as in gas phase processes.
In the course of their research developments, the applicants have now found a new method for increasing up to an unexpected level the polymerisation activity of a Ziegler-Natta type catalyst during the (co-)polymerisation of olefins using said Ziegler-Natta type catalyst, in particular during the gas-phase (co-)polymerisation of olefins in a fluidised bed reactor using said Ziegler-Natta type catalyst, especially during the gas-phase (co-)polymerisation of ethylene in a fluidised bed reactor using said Ziegler-Natta type catalyst. What is also unexpected from these huge activity increases is that the present method is absolutely not detrimental and rather beneficial to a normal and efficient process behaviour of industrial plants; in this respect, the Applicants have found that their method can be successfully applied for increasing plant throughput while avoiding the usual fouling problems the man skilled in the art would expect to face at these high activities.
Simultaneously, the applicants have found that this new method allows to increase the comonomer response during the copolymerisation of olefins using a Ziegler-Natta type catalyst, in particular during the gas-phase copolymerisation of olefins in a fluidised bed reactor using a Ziegler-Natta type catalyst, especially during the gas-phase copolymerisation of ethylene with another olefin in a fluidised bed reactor using said Ziegler-Natta type catalyst
This method is especially valuable for the industrial plants which will be now able by keeping their actual Ziegler-Natta type catalyst to increase significantly their polymer production.